The present invention is directed to an imaging device for a printing form, having at least one light source and one imaging optics for projecting the at least one light source onto the printing form, at least one optical element of the imaging optics being movable by way of an actuator element.
When printing forms, printing-form precursors, so-called masters or the like (all referred to in the following, in short, as printing forms) are imaged, whether it be in a printing-form imagesetter or in a printing unit of a printing press, using one or more light beams, which are projected by an imaging optics onto the surface of the printing form, it is necessary to bring one partial area of the printing form surface to be imaged into correspondence with the image area of the imaging optics in such a way that the focus of the light beam or the foci of the light beams is/are substantially disposed on the surface of the printing form, to enable energy to be supplied in a controlled manner, in particular with a defined intensity, to a specific surface. This objective is often achieved by shifting the focus of the imaging optics, in that the position of an optical element, in particular of a lens, is varied in the imaging optics.
From U.S. Pat. Nos. 5,146,242 and 5,248,992, an imaging device is known, for example, whose imaging optics has a variable focus. A plunger coil actuator makes it possible to change the position or location of a lens in such a way that it is displaced along the optical axis of the imaging optics, the focus of the imaging optics being displaced in the process. The light emitted by the imaging optics and directed at the surface of an object to be imaged is measured as it is reflected back into the imaging optics, so that a control signal or feedback control signal can be generated for the plunger coil actuator.
The prior German Patent Application No. DE 102 33 491.9, respectively, the prior U.S. Patent Publication No. 2004/0136094 describe another imaging device having a variable focus for a printing form. These two documents are hereby incorporated in their entirety by reference herein. The imaging device includes an imaging optics having a number of lenses positioned along an optical axis, through which the optical path from a light source, in particular from an array of individually addressable laser diodes, runs at least twice to a printing form. After passing through the lenses, the optical path is folded by a mirror, so that the light propagates through the lenses once more, in particular in the opposite direction. At least one of the lenses is designed to be movable along the optical axis. The imaging device includes a device, for example in the form of a triangulation sensor, for determining the distance between the imaging device and the printing form. On the basis of a distance measurement, the position of the movable lens can be controlled or regulated in such a way that the focus of the imaging optics is situated on the surface of the printing form to be imaged.
Since the surface of a printing form having a multiplicity of image spots is imaged in a short period of time, a highly dynamic, but, at the same time, very precise actuator (typically more precise in the micrometer range) must be used for positioning and guiding an optical element, in particular a lens, to enable the position of the optical element to be adjusted very quickly and exactly. An actuator of this kind does, in fact, make possible a correct displacement by a specific amount in relation to the just assumed actual position of the optical element, however, the displacement can only be precise to the extent that the actual position of the optical element is also already correct for the imaging operation.